miller



Jan. 24, 1956 H. B. MILLER CYLINDRICAL ELECTROMECHANICAL TRANSDUCER 2 Sheets-Sheet 2 Filed May 5, 1950 FREQUENCY IN KILDCYCLES FIG, 2

FREQUENCY IN KILOCYCLES FIG. 3

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INVENTOR. HARRY B.M|LLER BY ATTORNEY United States Patent CYLINDRICAL ELECTROMECHANICAL TRANSDUCER Harry B. Miller, Cleveland Heights, Ohio, assignor, by mesne assignments, to Clevite Corporation, Cleveland, Ohio, a corporation of Ohio Application May 5, 1950, Serial No. 160,365 14 Claims. (Cl. 340-8) This invention relates to a transducer for operation in a medium 'such as water and for. transducing therein between electrical and mechanical types 'of energy. Specifically, the transducer is useful for translating electrical energy into mechanical energy or for translating inechanical energy into electrical energy.

Transducers of the type mentioned above are widely used for translating electrical energy into mechanical energy or vice versa. Specifically, piezoelectric materials and magnetostrictive materials have. been used for this purpose. More recently barium titanate, when treated in a particular manner, has been found to ..have transducingproperties which are tions.

Also, it has been known for some time that a cylinder has desirable transducing characteristics for certain applications. It is diflicult, however, to provide .a cylindrical transducer with the piezoelectric crystals of the prior art for the reasons that, due to their characteristics of growth, it is impossible to provide a cylindricaltransducer which; moves radially at all points. "Such a device can be approximated by the use .of single crystals if 'a large mosaic of crystals is used but this provides a very expensive structure. The use of magnetostrictive materials for the same purpose also has certain inherent disadvantages. On the other hand, it is known that polycrystalline barium titanate which has been vitrified and treated in a manner to provide transducing properties can be made to have a radial action at all, points on the surface when in a cylindrical form. The use of a transducer in a liquid such as water, .for example, can provide a characteristic which is symmetrical at all points around the surface of the cylinder and in a plane normal to the axis of the cylinder. It, therefore, becomes very desirable in many applications to utilize a tube of electromechanically sensitive dielectric material of the-nature of barium titanate to provide a transducing action in a liquid,-either for changing electrical energy to mechanical energy, or acoustical energy in the liquid, -or for changing such acoustical energy in the liquid into an electrical form.

quite beneficial in certain applica- 2 ,732,536 Patented Jan. 24, 1956 hollow "cylindrical transducer for operation within a liquid with the outside surface of the cylinder acoustically exposed to the liquid and with the inside surface of the cylinder substantially acoustically shielded from the liquid and in which the dimensions of the cylinder are proportioned to provide a relatively uniform response over a very wide range of frequencies.

In accordance with the invention a transducer for operation in a medium and for transducing therein between electrical and mechanical types ofenergy comprises a hollow cylinder having a'len'gth not substantially greater than half its circumferencejand of electromechanically In the copending application Serial Number 154,037,

tiled April 5, 1950, in the name of Robert A. Langevin and assigned to the same assignee as the present invention, there is described a transducer of'the general type here under consideration which provides certain advantages by the use of a hollow cylindrical transducer element which has a ratio of wall thickness to diameter chosen in a specific manner. Applicant has discovered that, for certainapplications of a device of the general type here under consideration, decided advantages can be obtained when the length of the cylinder is proportioned with reference to the diameter or circumference of the cylinder in a particular. manner.

It is an object ofthe invention to provide an improved transducer for use. in changing electrical energy to mechanical energy, or vice-versa.

It is stillanother object of the invention to provide a transducer in the form of a hollow cylinder and which has a substantially improved transducing characteristic over a very wide range of frequencies.

it is. a specific object of .the' invention to provide a sensitive material. This transducer also comprises means, including electrodes on. the cylinder-and provisions for causing one surface of the cylinder to be acoustically exposed to the above-mentioned medium and the other surface of the cylinder to be acoustically shielded from the medium when the cylinder is immersed in the medium, for applying one of the above-mentioned types of energy to the cylinder z tndv for deriving the ther above-mentioned types of energy from the cylinder.

For abetter understanding of the invention together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings and its scope will be pointed out in the appended claims. Fig. 1 of the drawings illustrates a transducer in accordance with the invention which can be utilized either for translating electrical energy to mechanical energy, or for translating mechanical energy to electrical energy; Fig. 2 represents afrequency-response characteristic of a transducer used as a microphone and of the general type under consideration and proportioned inaccordance with the teaching of the prior art; Fig. 3 shows the frequency-response characteristic of a device proportioned in accordance with the teachings of the present invention; Fig. 4 illustrates an underwater sound microphone which is constructed in accordance with the teaching of the invention; and Figs. 5 and 6 illustrate still another embodiment of 'an underwater microphone constructed in accordance with the teaching of the invention.

Fig. 1 of the drawings illustrates a preferred embodiment of the invention. Here a transducer 10 is illustrated in the form of a 'hollow cylinder of electromechanically sensitive dielectric-material which may be barium titanate. This cylindcrhas an electrode 11 on,the outside surface and an electrode 12 on theinside surface thereof and, when in operation, is adapted to be immersed in a liquid such as water- 13'. The arrangement includes means for causing the outer surface of-the cylinder to be acoustically exposed to the water 13 and for causing the inner surface of the cylinder to be acoustically shielded from the water when the cylinder is immersed in the water. Specifically, plugs 15 and 15 are provided for the cylinder and' 'these' plugs may b'ecomprised of cork, rubber, or any other suitable material. An electrical generator 16 is shown for applying electrical energy to the electrodes 11 and 12. ,Also, amicrophone 17 is included in the liquid 13 in order to generate electrical currents corresponding to the mechanical energy introduced into the liquid by the operation of the cylinder 10 as a transducer for converting the electrical energy of generator 16 to mechanical energy, or specifically to sound energy, in the water 13. This "sound energy, of course, may be very much above any audible frequency. A reproducer 19 is illustrated for reproducing or utilizing the energy from the transducer or microphone 17. The reproducer 19 could, for example, be a cathode-ray tube or a loud speaker or any other reproducing-device depending upon the purpose for which the apparatus is to be used.

In considering the operation of the device of Fig. 1 as .so far described, it will be seen that electrical energy gene'rated'by the 'g'eneratorlli is applied to the electrodes 11 and -12 therebycausing, 'under the conditions to be described in more detail hereinafter, a mechanical action in the cylinder 10 which, in turn, causes sound waves .to be propagated 'ii'nto' the'water 713. These sound .waves are picked up by the microphone 17 and reproduced on the reproducer 19in anyides'ired'manner. .As'descrlibedtheretoregtlre arrangement dffFig. "1 is one ffor changing an-electrical input to transducer '10 into a mechanical output which -comprises the sound energy propagated into the water 13.

Alternatively, the arrangement 'of'Fig. '1 can he used in thereverse manner. In order to illustrate this use of the arrangement, switches-.22 and 23 are shown. When these "switches are in their dotted position, the transducer 17 is connected to a generator T24 and the electrodes 11 and 12Jare connected to .a reprodu'cergZS. Under these conditions, the generator T24 is .effective to excite the deviceI7 and produce sound energyfinftheliquid 13. This soundenergy impinging on the cylinder 11.0'is-transformed by the cylinder I 'intoelectrical energy and means, including electrodes '11 and 12, are v provided for deriving and utilizing this electrical energy. Specifically, Ltheelec- 'tri'cal energy'is reproduced'by.reproducerZS which again may be an-oscillograph for any other device which can be actuated by the electrcialtenergy.

.In order to explainin detail applicantisnovel method of proportioningthetransducer 10, it is deemed desirable to assume that the device is operated withithe switches '22 and 23 in the dotted position. One .suchrcylinder comprised of commercially availableibarium .titanate, .thatis substantially purebarium .titanate except for commercial impurities, and prepared in the .manner indicated in detail in the above-mentioned 'Langevin application, provided afrequency response "characteristics as illustrated in Fig.2. This-ceramicelement was 6".long hadan outside diameter of 1 /2" and .a wall .thickness of /8". When connected as described, devices of the general type here under consideration are .suita'ble for use as underwater listening devices, .for example, for .detectiono'f.submarines. Fig. 2 shows the frequency response characteristic of the device under consideration. It will beseen that the response characteristic .Iis substantially :fiat in thelower portion of the range butthattthere -isadecidedzdip at about kilocycles and aresonantresponse at about 13 kilocycles. Thereis anradditional resonant response at about 40 -;kilocycles. Jnasmuch .as .for .many applications, specifically for use .in applications for detecting submarines, it is es sential that such apparatus.havearesponse which is flat over a very wide frequency range, the characteristic of Fig. 12 'is .totallyinadequate for .thepurpose. ltis desirable "for applications .in most underwater listening .devices of .the .type .here under consideration that the apparatus have a response which .is substantially ifiat'toatleast :30 kilocycles anda flat response to 40:0reven'60kilocycles is frequently required. Applicanthasprovided aresponse of this type'lby .the apparatus illustrated in Fig. ,3.

InFig. 3 there are provided three titanate .tubes 30, 31,

and .32 of the -type'here under consideration. :Each tube is electroded .on its :inside ,and outside surface. These tubes correspond totube v10 of fig. 1. These hollow cylinders are proportioned .to-have adength'which is not substantially greater than half ,the .mean circumference for a purpose which will be explained :in'more :detail later. .The cylinders 30, .3-1, and 32 are held in'an-axial aligned position by means of .metalrod 33 through the cylinders. The rod-33 clamps the cylinders by means of .nuts .34, 35 and pieces 3.6 *and -37 -which, tor example, may .be .of .a .phenolic condensate such, as Bakelite. Rubber .washers 3.8, 39, 40 and 41 ar.c provided-;at :the ends of .thetubes .in order-toprevent the tubesitrorn being rigidly .clarnped .and -.'to allow the slight end motion in each casewhich is-necessary for the ;-present -:purposes. The -tubes -30, .31 ,and .32 are {electrically connected in series by n eans ofzleads=43 and-4:4 and byzmeans-of a connection 45 between the outside electrode of cylinder '32 'and "the inside electrode 'of cylinder 3'1 and connector 46 between the outside electrode of cylinder 31 and the inside electrode of cylinder 30. The lead 43 is connected to the inside electrode of cylinder 32 and the lead 44 is connected to the outside electrode of cylinder 3.0 by means of a connector 47. Alternatively, if desired, the tubes 30, 31, and 32 may-beconnected inparallel.

Arstring or c0rd'48is provided between an eye .inone end of the rod 33 and the leads =43 :and "44 ;in order-to prevent tension from being applied .to .the pointsat which the leads "43 and 44 are connected when the device is carried or handled by'me'ans o'fthe leads 43 or 44.

The three-cylinder device :just described may be .connected directly .in "place of ithe single-cylinder device :in thecircuit ot.Fig. l. Iheidevicemf Fig. 3 was-found to have the characteristic :as illustrated .infiig. --4 .when operated in water under the same conditions described above with reference to Fig. :1. Here it will bee-seen that the response :is relatively flat to approximately 40 :kilocycles and that :a decided'improvementzin the :response has been eflected.

While applicant "does-not wish to be'limited'to any particular :theory :of :operation of :the "device, it is thought that the .reason ;for ithe1differencein the response characteristicgfor'the Fig. '1 :device and for the'Fig. device is as follows. The single tube illustrated in Fig. l and under the :conditions described has two principal resonant characteristics. -.One ofithese corresponds toa length mode of :resonance and .the other corresponds to a circumferential mode of resonance. Thetfrequency of the length modeiof .resonancevaries inversely with the length of :the "tube .involved and the circumferential mode of resonance varies :inversely as half the circumference. Therefore, it appears thatrthe length resonance of the Fig. 1 device ,is represented :by the'hump in the characteristic of.Fig. 2 at about 13 kilocycles, and that the circumferential resonance is represented "by the hump at=about 40 kilocycles. The length resonance is preceded bya strong dip atabout F10 kilocycles,probabl y for thereason that the titanate material under consideration tends to expand both .in length and in circumference-due to electrical excitation and tends to contract in circumference while expanding mechanically due to its mechanical action, resulting in .a phase relationship between the electromechanical activity and the pure mechanical activity which produces "the-dip at 1'0 kilocycles. In any event, the device-of :Fig. 3 appears-to have its length resonance at about '60 kilocycle's, so that it is above the circumferential resonance'which lies at about l'o'kilocycles. Under these conditions, the response characteristic of Fig 4 is provided-resulting in a very greatiimprovem'ent in the opera- ;tion of the device.

Actually, the proportioning of the ceramic cylindcrs is not critical 'so' long as the length of each individual tube is not substantially 'greater thau half the outside circumference of ithe tube. in the device illustrated in Fig. '3, the length of each tllbfi'iS-SlibSiflfitiflilY equal to-two thirds of half ithe circumference. lt hasbeen found that very good resultscan be provided if the length is exactly equal to orjust slightlymore than half thecircumterence of the tube. Such a proportioning -provides a tube which has only aslight hump in the vicinity-ofihe lower 'mode and a somewhat stronger 'response at the higher resonance mode with no dip whate'-.'er between=thetwo resonant responses. Such a device has been provided utilizing the teachings described herein which has a-strong response to about- 45 kilocycles, which is substantially fiat to about 35 kiiocycles, and'which has-nostrong 'dips'present inthe response characteristic.

The device just described-may be utilized'as a'listening device for underwater sounds and, -alternatively,'whenthe switches'are-in the full line'pcsition illustrated in Fig. '1, the device may be utilizedasagenerator ot underwater sounds. 'EII! either. case the .responsccharacteristic is substantially uniform around meals of the tubes for each plane normal to the axis. The response is strongest for the direction perpendicular to the axis of the cylinder. The directional lobe of the device just described becomes sharper as the length of the tube becomes longer. Thus a very long structure can be provided by the use of additional ceramic tubes which is very directional normal to the axis, that is, the principles of-the invention can.be utilized by dividing the length of the electro-mechanically responsive element into sections as taught in connection with Fig. 3.

Actually, in a long-line device f the type under consideration, it is not necessary to provide a substantially continuous tube of electromechanically responsive material as illustrated in Fig. 3. Specifically, in the long-line device, the individual tubes-can have a substantial spacing therebetween as illustrated in Figs. 5 and 6; Thefide vice of Figs. 5 and 6 comprises tubes of electromechanically sensitive material 50, 51, etc., having a substantial spacing between the ends of the .tubes. The device illustrated may be of any length and one end is illustrated in Fig. 5 and the other end is illustrated in Fig; 6 In the device under consideration, the electromechanically sensi tive elements are clamped on a metallic tube'53 by means of metal rings 54, each of which have a threaded hole 55 for receiving a set screw in order to hold the rings 54 and the tubes 5t), 51, etc., in place. The ends of the tubes 50, 51, etc., are clamped against the rings 54 by means of rubber washers 57. The outside electrode of the ceramic tubes 50, 51, etc., are connected together by means of leads 59 and the inside electrodes of the ceramic tubes are connected to the metal tube 53 by means of connectors 60, 61, etc. The tubes 50, 51, etceare thus connected in parallel and the terminal connections for the device are not illustrated. The device of Figs. 5 and 6 is preferably oilfilled and, for the purpose of retaining the oil around the device, there is provided a housing which comprises metallic end sections 62 and 63 adapted to be clamped to the tube 53 by means of set screws in the threaded holes 64 and 65. A fitting 66 to receive an electrical connector for the device is provided and an acoustically transparent rubberhousing or tube 67 between the end plugs 62 and 63 effectively retains the oil inside. The housing 67 is held on the end plugs 62 and 63 by means of clamps 68 and 69. In order to allowfree access of the oil to all parts of the device, holes 70, 71, 72 and 73 are provided in the tube 53 but a pressure-release covering 74 is provided for the inside of each of the cylinders. This covering may be of closed-cell sponge rubber. A plug is inserted in the threaded hole 75 and the'connector containing the leads is inserted in the threaded hole 76 in the member 66 to hold the oil inside the device. Threaded holes 77 and 78 are provided for making connections (not shown) with the leads to the electrodes on electromechanically sensitive tubes 50 and 51, etc.

In considering the operation of the device of Figs. 5 and 6, it will first be understood that its operation is generally similar to that of Fig. 3 and that it has the same general type of response characteristic. However, the directional response characteristic of the deviceof Figs. 5 and 6 is very much sharper .due to its increased length. This sharp response characteristic is retained even though the'mechanically sensitive cylinders 50 and 51,-etc.,have a substantial space between them. Actually, it has been found that the spacing of the cylinders 50, 51, etc., does not substantially upset the sharp-response characteristic over a wide range of wave-lengths providing that the spacing between the cylinders, center to center, is not substantially greater than A of the shortest wave-length in the range. This spacing of the shortestwave-length is, however, rather critical and if the spacing between the tubes 50, 51, etc., is increased. to any extent beyond this amount the sharp directional pattern 'imniediat'ely breaks up into a series of narrow spikesand the response of the device becomes very much inferior for most applications.

While there has been described what are at present considered to be the preferred embodiment of the invention, it will be obvious to those'skilled in the art that various changes and modifications may be made therein without departing from the invention and it is aimed in the appended claims to cover all such changes and modifica tions 'as fall within the true spirit and scope of the invention.

What is claimed is:

l. A transducer for operation in a medium and for transducing therein between electrical and mechanical types of energy comprising: a hollow cylinder having a lengfli not substantially greater than half its circumference and of electromechanically sensitive material; and means, including electrodes on said cylinder and provisions for causing one surface of said cylinder to be acoustically exposed to said'medium and the other surface of saidcylinder to be acoustically shielded from said medium when said cylinder is immersed in said medium, for applying one of said types of energy to said cylinder and for deriving the other of saidtypes therefrom.

2. A transducer for operation in a medium and for transducing therein between electrical and mechanical types of energy comprising: a hollow cylinder having a length not substantially greater than half its circumference and of electromechanically sensitive material; and means, including electrodes on said cylinder and provisions for causing the outer surface of said cylinder to be acoustically exposed to said medium and the inner surface of said cylinder to be acoustically shielded from said medium when said cylinder is immersed in said medium, for apply- I ing one 'of said types of energy to said cylinder and for deriving the other of said types therefrom.

3. A transducer for operation in a medium and for transducing therein mechanical energy to electrical energy comprising: a hollow cylinder having a length not substantially greater than half its circumference and of electromechanically sensitive material; means including provisions for causing the outer surface of said cylinder to be acoustically exposed to said medium and the inner surface of said cylinder to be acoustically shielded from said medium when said cylinder is immersed in said medium so that sound waves in said medium are etfective to apply mechanical energy to said cylinder; and means comprising electrodes on said cylinder for deriving electrical energy from said cylinder in response to the application of said mechanical energy to said cylinder.

4. A transducer for operation in a medium and for transducing therein electrical energy to mechanical energy comprising: a hollow cylinder having a length not substantially greater than half its circumference and of electromechanically sensitive materialymeans including electrodes on said cylinder for applying electrical energy to saidcylinder; and means including provisions for causing the outer surface of said cylinder to be acoustically exposed to said medium and the inner surface of said cylinder to be acoustically shielded from said medium when said medium is immersed in said water so that mechanical energy is propagated into said water in the form of sound waves in rcsponse'to the application of said electrical energy to said electrodes.

5. A transducer for operation in a'medium and for transducing therein between electrical and mechanical types of energy comprising: a hollow cylinder having a length substantially equal to half its circumference and of electromechanically sensitivematerial having the property of expansions and contractions around its circumferential directions due to its electromechanical action when in operation which are associated with corresponding expansions and contractions in its length directions; and means, including electrodes on said cylinder and provisions for causing one surface of said cylinder to be acoustically exposed to said medium and the other surface of said cylinderto be acoustically shielded from said medium when said cylinder is immersed in said medium,

for applying .one of said ltypes l of :energy to sa'id cylinder and for deriving tthe other iof said types therefrom.

6. A transducer for operation in 'a 'medium and "for transducing: therein 'mechanical energy to electrical energy comprising: :a hollow cylinder having a length substantially equal ito half its circumference and of electromechanically sensitive :material "having the properties of expansions and contractions around its circumferential directions due to its electromechanical :action when in operation which are associated withcorresponding expansions :and contractions in its length directions; :means including provisions 'for causing the 'outer :surface of said cylinder to be acoustically exposed to said :medium and theinnersurfaceof saidtcylinder'to'be acoustically'shielded from ssaid'medium whensaid 'cylinder is immersed in said medium for applying mechanical .energy in the form of sound waves' to said cylinderwand-rneans including electrodeson said cylinder 'forderiving-electrical energy from said cylinder in :response to the application .of said mechanical :energy to said cylinder.

7. A transducer for operation in a medium and 'for transducing therein electrical energy to mechanical energy comprising: a'hollowicylinder having a length substantially equal to ihalf its circumference and ef electromechanically sensitive'material having the'property of expansions and contractions around its circumferentialdirections due to its.electromechanicalaction when in operation which are associated-with'correspondingrexpansions and contractions in its .lengthldirections; means including electrodes on said cylinder for-applying electrical energy to said cylinder; and means iincludingxprovisions for causing-the outer surface :of said cylinder to be acoustically exposed to said medium and theinn'ersurface ioffsaidrcylinder to be: acoustically shielded from :saidamedium when said cylinder is immersed .insaid'medium for :applying' mechanical energy in .the'form or" sound waves ;to.said medium :in response to the applicationofsaid electricaltenergy to said cylinder.

8. .A transducer ifOI :operation in .a medium and for transducing ztherein between electrical :and mechanical typeset energy :comprising: 'aiplurality'of similar axially aligned :hollow :cyllinders each .having a length not substantially greater :than half .its circumference; and means, including electrodes on each of "said cylinders and provisions :for causing .a corresponding surface of each of said .cylindersito :bezacoustically exposed to said'medium and .the other corresponding surfacestof each of saidcylinders to be acoustically shielded"from said medium when 'said transducer is immersedinsaidmedium, for applying one .of said types of .energyto .each of said cylinders and for deriving-the other of said :types therefrom.

.9. A transducer for "operation in a medium and for -transducing itherein between electrical and mehanical types-.of .energy:comprising: a plurality iof=similar axially :aligned hollow: cylinders each'rhaving ailength not substantially greater athan half its circumference; and -means including electrodes .on :each of 'said cylinders and pro- .visions .for causing the outer surface :of each of said cylinders to .be acoustically exposed to said medium and the inner surface of each of :said cylinders to be acous- .tically shielded :from said medium when said transducer is immersed in said medium, for :applying one o'f said types-of energy to each rot-said cylinders and for deriving the-other of said types therefrom.

510. A transducer for operation in 'a'medium and for transducing therein mechanical energytoclectrical energy comprising: aplurality of axially aligned :hollow cylinders each having a length not substantially greater than its :circumferenceymeans including provisions forcausingthe :outersurface of each 'o'f=sai'd-"cylinders to be acoustically exposed :to said medium and :-the :inner surface .of each of said cylinders to :be :acoustieally shielded from said :8 trial (energy t'fmm said uransducer :in :response tie the applicationrefrsaidnnechanical energyzto sa'idcylinders.

1111.95 transducer :for operation :in :a medium "and for .transdudin-g dherein aelectricalrenergyzto :mechanical :energy comprising: a :plurality :of :similar :axially aligned hollow cylinders -.ea'ch ihaving in length :not :substantially greater thanzimrcircumferenceymeans including 'clectrodeson each (if ssaid'rcylind'ers rfor applying ielectrical energy ato said cylinders; and :means iincluding :provisions for causing the:outerisurfacerofteach of'said cylindersrtozhe acoustically exposed to said medium and the inner surface 10f each of :said :cylinders ato 'Lb'e i-acoustically shielded from said medium'zuihenssai'dtransducerrisimmersed in said medium for:causingzmechanicalenergyxini the :fornro'f sound' waves to .be ipropagatcd into 'said medium in response to the application ro'fssaid electricalcnergy to :said cylinders.

.12. .A transducer for "operation-in 'a medium and. for transdueing :thcrein between :electrical and :mechanical typessof :energymomprising: :a plurality :of axially aligned similar rh'ollow :cylinders -:each having a =length substantially equal to half :its circumference :and of electromechanically sensitive :material having ;the property of expansions rand ccontractions around sits circumferential directions duez-toiits electromechanical action when inoperation which are associated :with corresponding expansions and Icontra'ctionsiin'itslengh directions; and means, ineluding electrodes ton :said :cylinders and provisions for causing 1a correspondingssurface-of each of said cylinders to rbeacoustically .exposed .to 'said medium and the other corresponding surface of each :of .said cylinders to .be acoustically:shieldedimm said medium when said transduceriis :immersed in zsaid zmediumgfor applying one of said types of emergyrto .'said :cylinder and :for deriving the other aofssaid types :therefrom.

2.13.1AitIZ3HSdHCBI' fol-operation in a medium and for .transducing :therein ibetween electrical and mechanical acoustically shielded from said medium when said transducer is immersed in :saidmedium, for applying one of said itypes of energy to said cylinder and for deriving the :other of said ztypes therefrom.

l1 4. .Aitransduceriforoperation in a medium and for transducing ztherein between electrical and mechanical typessof energynvera givenzranger'o'fwave-length; a plurality rof -similar axially :aligned zhollow cylinders each having a length not substantially greater than half its circumference and :being spaced from any :of the other ofisaidtcylindersabyaidistance not substantially greater than three-fourths \of the shortest wave-length in said range and o'flelectromechanically isensitive material hav ing Tthfi property of :expansions and contractions around its circumferential:directions due zto :its electromechanical action when inroperation'which-are associated with conespendingexpansions-and contractions in its length directions; :and means, :including "electrodes on said cylinders andprovisions for causing the outer surface of each of saidrcylinders to the :acoustically exposed to saidmedium and :the .innerr'suiface 'of said cylinder to be acoustically shielded from :said medium :when said transducer is intmersediin said :medium, for applying one of said types of energy :tozsaid-ccylinder sand-for deriving the other of said :types ztherefrom.

References .Cited fin'thc file of this I patent UNITED STATES .PATENTS 2 ,4D2;51-5 Wainer... .-Iune .18, i946 

